In 1927, the German Army adopted the Enigma machine.
After several experiments and improvements with a Steckerbrett
(plug board), they finally introduced the Enigma I
for operational use by the Wehrmacht and Luftwaffe in 1932.
In 1934, the Kriegsmarine (Navy) followed with the introduction
of the M1 (Ch. 11g); a machine that was functionally compatible with the
Enigma I (Ch. 11f).
There were, however, some (minor) manufacturing differences.

For example, the wheels had letters (A-Z) on their circumference rather than
numbers (01-26) and the machines had a 4V power socket that was suitable for
use aboard a ship or U-boat.

About 611 M1 units were built.
The M1 was followed in 1938 by the M2,
of which 890 units were delevered. Finally, in 1940, it was replaced
by the M3,
of which approx. 800 units were built [2].
All three machines, M1, M2 and M3, had the same internal designator
Ch. 11g. We will therefore use the name M3 (which refers to the
procedure) to identify these three machines.

The image above shows an Enigma M3 aboard German U-Boot U-124.
The machine is located in a recessed bay in the bottom left of the image.
In the enlargement, a power cable
is visible in the corner of the bay. It was used to
supply 4V directly to the machine.
The photograph was taken in March 1942, nearly a year before it was replaced
by the M4.
Also visible in the picture is a coding table to the left of the Enigma
(bigram table?) and a message form (Funkspruch) at the far right.

Initially, the M3 was supplied with 5 cipher wheels that were
compatible with the wheels supplied with the Enigma I.
This way, all three departments of the German Army could exchange
messages.
In 1939, three more wheels were added (VI, VII and VIII), which were used
exclusively by the Kriegsmarine.
At the start of WWII, the U-Boot section of the Kriegsmarine
also used the Enigma M3, until it was replaced by the 4-wheel
Enigma M4 in February 1942.

What's in a name?

It is sometimes thought that the name Enigma M3 refers to a 3-wheel
machine and that for the same reason M4 was used for the 4-wheel
model. However, this is not correct. It was just the name of the key
procedure that was in use when the machine was issued.
The range started with M1 and ended with M4, the special U-Boot key.

Extra wheels VI-VIII

Whereas the Enigma machines of the German Wehrmacht
(Heer and Luftwaffe) were supplied with 5 cipher wheels, all Naval
machines had 8 wheels to choose from. The first five wheels
(I-V) were identical to the five wheels supplied to the rest of the
Army, allowing some level of compatibility,
but the additional three wheels (VI-VIII) were used
exclusively by the Kriegsmarine (Navy).

Of the 8 wheels supplied with the Enigma M3, three would be placed
in the machine at any time, subject to the current key settings.
Although 3-of-8 wheels theoretically gives a total number of wheel
orders of 336 (8 x 7 x 6), this was limited in practice by the operational
procedures. There were instruction that one of the 3 wheels in the
machine had to be an extra wheel (VI-VIII) and that a particular
extra wheel could not be used in the same position on two successive days.

The remaining 5 (unused) wheels were stored in a special
wooden box. The image on the right shows such a box with the
unused wheels in it.

The box has space for 7 wheels but only 5 positions were used.
The remaining positions were probably meant for a future
extra wheels or to store additional gadgets, such as UKW-D.

More pictures of this box below.
Click any of the images to enlarge. Note that the serial number
of the wheels is present on the box' ID-plate and is also engraved
in the lock of the wooden box.

Wheels I to V each have one turnover notch, but the three extra
Naval wheels (VI-VIII) each have two notches.
This was done to generate more frequent wheel-turnovers and, hence,
make wheel stepping less predictable. The problem however,
is that the cipher period is reduced, as 26 (letters) is dividable
by 2 (notches).
As you can see in the images below, these
two notches are positioned
opposite each other. Details of the wheels wiring can be found
in the table below.

The construction of the Naval wheels is different from other
Enigma wheels. Rather than using a spring-loaded pin to set the
Ringstellung (common on all other wheels), the user has to press
two pawls simultaneously.
Furthermore, the wheels are marked with the 26 letters of the
alphabet (A-Z) rather than numbers (01-26).
The overall dimensions of the wheels are identical however.

Wheel turnover

Please note that althoug the Zustzwalze can be set to any position,
it does not move during the ciphering process. In other words:
it can't be moved by the wheel to the right of it. This could be
considered a weakness as it limits the possible number of permutations
of the entire system. The combination UKW+Zusatzwalze can
be regarded as a selector between 26 different UKWs.

Another weakness of the wheelturnover mechanism is that the wheels
move regularly. Only after the rightmost wheel has completed a
full revolution, does it cause the next wheel to make a single step.
As a result, the 2nd wheel (from the right) will only make one step
every 26 characters and the 3rd wheel will hardly ever move.
This makes the cipher period predictable and easier to break.
The only machine that did not suffer from regular stepping was
the Enigma G (Zählwerkmaschine).

The three extra wheels (VI, VII and VIII) have two notches each,
which causes a more frequent wheel turnover and less regular stepping.
This was done as the 3rd wheel (from the right) hardly ever moved
during the ciphering process.

Note however, that the wheels have 2 notches which is not a
relative prime of 26 (26 can be divided by 2) and that the notches
are positioned oposite each other
(see the image on the right).
The result is that the cipher period is effectively halved,
which was yet another weakness of the system; a bonus
for the codebreakers.

Furthermore, operational procedures instructed the use of at least one
of the extra 3 Naval wheels (VI-VIII) every day, and that such a
wheel could not be used in the same position on two successive days.
This was known by the code breakers at
Bletchley Park
and reduced the number of possible permutations.
In fact, it made guessing the wheel-order easier.

Wheel wiring

The wiring of the M3 is given in the table below. Wheels I thru V
are identical to those of the Enigma I. The same is true for
UKW B and C. The three additional wheels (VI, VII and VIII) were
used exclusively by the Kriegsmarine.